Also, in terms of the rate at which an enzyme acts, other than substrate concentration and reaction rate, are there any other conditions that would regulate the rate at which an enzyme works? Are there such thing as inherently "slower" or "faster" enzymes?

"Also, in terms of the rate at which an enzyme acts, other than substrate concentration and reaction rate, are there any other conditions that would regulate the rate at which an enzyme works? Are there such thing as inherently "slower" or "faster" enzymes?"

Oh my goodness yes yes yes. Different protein structures have different rates of catalysis - a single amino acid change can alter or eliminate activity. Temperature and pH affect rate. Many enzymes are controllable by other enzymes -- look at allosteric regulation:http://en.wikipedia.org/wiki/Allosteric

Assuming you mean regulation of rate of one enzyme, then you're interested in things such as posttranslational regulation (phosphorylation). As jonmoulton mentioned, allosteric regulators are important.

What I meant is the regulation of enzyme produced by one gene. On the other hand, as jonmoulton wrote, you can have several genes and the respective proteins will have different amino acids in active side, they may be localized in different compartments etc. But I do not consider this much as regulation, because you must have the gene already, so it's not really something you can regulate, right?